1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a color filter substrate for a liquid crystal display device and a fabricating method thereof.
2. Discussion of the Related Art
With the rapid development in the information technology field, display devices have evolved to be able to process and display increasingly large amounts of information. Flat panel display technologies recently developed for display devices have reduced thickness, light weight, and low power consumption. Among these technologies, liquid crystal display (LCD) devices have been used in notebook computers and desktop computer monitors due to their superior image resolution, color image display, and image quality
In general, an LCD device includes an upper substrate, a lower substrate, and a liquid crystal layer disposed between the upper and lower substrates. The LCD device makes use of the optical anisotropy of liquid crystal material and produces images by varying the light transmittance according to the alignment of liquid crystal molecules using an electric field.
The lower substrate, is commonly referred to as an array substrate, and includes thin film transistors and pixel electrodes. It is fabricated using repeated photolithographic processes to pattern thin films. The upper substrate, which is commonly referred to as a color filter substrate, includes a color filter layer for displaying color images. The color filter layer includes sub-color filters of red (R), green (G), and blue (B), and is formed by various methods including, for example, a dyeing method, an electro-deposition method, a pigment dispersion method, and a printing method. In general, the pigment dispersion method is more commonly used because it forms a fine pattern with good reproducibility.
FIGS. 1A to 1E are schematic cross sectional views illustrating a process of fabricating a color filter substrate for a liquid crystal display (LCD) device according to the related art. Here, the pigment dispersion method is used.
In FIG. 1A, a black matrix 15 is formed on a substrate 10 by depositing a metallic material or coating a resin, and patterning the metallic material or the resin through a photolithographic process. The black matrix 15 blocks light leakage caused by irregular operation of liquid crystal molecules within regions of an array substrate other than pixel electrodes (not shown).
In FIG. 1B, a color resist 17, which may be one of a red, green, or blue resist, (in this example a red one), is coated onto the substrate 10 including the black matrix 15 by spin coating. A mask 20 having a light-transmitting portion and a light-blocking portion is disposed over the red resist 17, and the red resist 17 is exposed to light through the mask 20. Here, the red resist 17 is shown to have a negative property, i.e., a portion of the red resist 17 that is not exposed to light is removed.
In FIG. 1C, the red resist 17 (of FIG. 1B) is developed, and a red sub-color filter 17a is formed. Then, the red sub-color filter 17a is cured and hardened.
In FIG. 1D, green and blue sub-color filters 17b and 17c are formed on the black matrix 15 through similar processes, as shown in FIGS. 1B and 1C.
In FIG. 1E, an overcoat layer 23 and a common electrode 25 are subsequently formed on the red, green and blue sub-color filters 17a, 17b, and 17c, respectively. The overcoat layer 23 protects the sub-color filters 17a, 17b, and 17c, and creates a flat top surface over them. The common electrode 25 is made of a transparent conductive material, such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO).
During the fabrication method of the color filter substrate using pigment dispersion the fabrication method is complicated and requires significant amounts of time and numerous pieces of equipment, because the color filter substrate is fabricated by repeated processes of coating, exposing, developing, and curing of a color resist,. To solve the above problem, a fabrication method of a color filter substrate using thermal imaging has been proposed.
FIGS. 2A to 2F are schematic cross sectional views illustrating a process of fabricating a color filter substrate using a thermal imaging method according to the related art.
In FIG. 2A, a black matrix 35 is formed on a substrate 30 by depositing a metallic material or coating a resin, and patterning the metallic material or the resin by a photolithographic process.
In FIG. 2B, a red transcription film 40 is disposed over the substrate 30 including the black matrix 35. The red transcription film 40 includes a supporting film 40a, a light-to-heat conversion (LTHC) film 40b, and a color filter film 40c. 
In FIG. 2C, the red transcription film 40 is adhered under vacuum to the substrate 30 without micro bubbles. A laser head 50, from which a laser beam is irradiated, is disposed over the red transcription film 40. Then, the laser beam is applied to the red transcription film 40 within an area where a red color filter pattern will be formed while the laser head 50 is moved along a straight line. In the portion of the red transcription film 40 exposed to the laser beam, the LTHC film 40b transforms light absorbed from the laser beam into thermal energy which is emitted. Accordingly, the color filter film 40c is transferred onto the substrate 30 due to the emitted thermal energy.
In FIG. 2D, after removing the red transcription film 40, a red sub-color filter 45a is formed between the adjacent black matrices 35 on the substrate 30.
In FIG. 2E, green and blue sub-color filters 45b and 45c are formed through the same process, as shown in FIGS. 2B to 2D. The substrate 30 having the sub-color filters 45a, 45b, and 45c is placed in a hardening furnace, and the sub-color filters 45a, 45b, and 45c are hardened. Three hardening steps for the sub-color filters 45a, 45b, and 45c may be performed after forming each sub-color filter to prevent conglomeration between sub-color filters. However, using three hardening steps may increase process time and production cost.
In FIG. 2F, an overcoat layer 47 is formed on the sub-color filters 45a, 45b, and 45c in order to protect the sub-color filters 45a, 45b, and 45c, and to create a flat top surface of the substrate 30. Next, a common electrode 55 is formed on the overcoat layer 47 by depositing a transparent conductive material, such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO).
FIG. 3 is a schematic perspective view illustrating a laser beam scanning step of a process of fabricating a color filter substrate using a thermal imaging method according to the related art.
In FIG. 3, a laser head 50 having a plurality of laser pixels 52 scans a substrate 30 along a transverse direction and irradiates a laser beam onto a transcription film 40 by alternately turning the laser ON and OFF. The plurality of laser pixels 52 are turned on in areas where sub-color filters are formed and the laser beam irradiates onto the transcription film 40 in these areas. The plurality of laser pixels 52 are turned off in the other areas. In FIG. 3, for example, the plurality of laser pixels 52 are turned on while the laser head 50 passes over a first area “I” where a red sub-color filter is formed. The plurality of laser pixels 52 are turned off while the laser head 50 passes over second and third areas “II” and “III” where green and blue sub-color filters are formed, respectively. The switching ON and OFF of the plurality of laser pixels 52 may be controlled by a computer according to the speed at which laser head 50 moves, or according to the stage of the fabrication process (not shown).
FIG. 4 is a schematic cross sectional view taken along a line “IV-IV” of FIG. 3. In FIG. 4, after removing a transcription film, a red sub-color filter 45a on a substrate 30 is obtained. The red sub-color filter 45a is formed by irradiating a laser beam onto a transcription film by alternately switching the laser ON and OFF. But the energy of the laser beam is not uniform when it is started and stopped. Moreover, heat is diffused in the light-to-heat conversion film of the transcription film. Accordingly, an edge portion “PA” of the red sub-color filter 45a has an indented shape after irradiation due to the non-uniformity of the laser beam and the thermal diffusion. In addition, the edge portion “PA” of the red sub-color filter 45a becomes more indented due to adhesion between the transcription film and the substrate 30 when the transcription film is removed.
Because the indented edge portion “PA” of the red sub-color filter 45a is shielded by a black matrix 35, the indented edge portion “PA” does not cause degradation of the LCD device. However, because an inspection apparatus adjusted to have a high inspection sensitivity classifies the red sub-color filter 45a having the indented edge portion “PA” as a bad one, a color filter substrate on which the red sub-color filter 45a having the indented edge portion “PA” is formed is rejected as a bad substrate. On the other hand, when the inspection apparatus is adjusted to have a low sensitivity, the inspection apparatus does not classify the red sub-color filter 45a having the indented edge portion “PA” as a bad one. However, with the sensitivity reduced, the inspection apparatus can not detect a sub-color filter that is actually bad. Accordingly, an exact yield and an exact inferiority ratio may be not obtained. Moreover, because the color filter substrate classified as a bad one would be re-inspected and re-classified, a loss of time and manpower result. Similar problems arise with the green and blue sub-color filters formed by a thermal imaging method.